Growth yields and the efficiency of oxidative phosphorylation during autotrophic growth of Paracoccus denitrificans on methanol and formate

Abstract

Paracoccus denitrificans was grown aerobically in chemostat or pH-auxostat cultures with methanol or formate as the limiting carbon source. The maximum growth yields (expressed in g biomass/mole), determined in methanol-limited cultures, on methanol, oxygen and carbon-dioxide were 13.4, 15.5 and 24.8 respectively. The maximum growth yields, determined in formate-limited cultures, on formate and oxygen were 2.9 and 10.5 respectively. Elemental analysis of these cells yielded a cell formula of C6H10.8N1.5O2.9. With the help of the known elemental cell composition an equation for material balances could be drawn up. The calculated yields on oxygen and CO2 (Y O 2 and Y CO 2) were in good agreement with the experimental values. Autotrophically grown cells on methanol possess three active energy conserving sites in the electron-transport chain. The overall P/O ratio (the amount of adenosine triphosphate (ATP) synthesized per atom oxygen consumed) is calculated to be 2.05 with the matching Y ATP max value of 3.8, which is lower than the theoretical value of 6.5. With only 2 phosphorylation sites impossible Y ATP max values would result. Autotrophic growth on formate yielded cells in the possesion of only two active energy conserving sites, most probably sites 1 and 2. Formate crosses the cytoplasmic membrane as free acid, which is equivalent to co-transport with one proton. Therefore transport of formate is energy requiring. With →H+/site ratios of 2 or 3 respectively 0.5 or 0.33 moles of ATP are necessary to transport 1 mole of formate. Consequently the P/O ratio in formate-grown cells is respectively 1.5 or 1.67 with the matching Y ATP max values of 3.1 or 3.5. Therefore during autotrophic growth on methanol and formate the same amount of adenosine triphosphate is necessary to synthesize 1 g of biomass. The 95% confidence intervals of the Y ATP max values overlap completely. The found Y ATP max is 50–60% of the theoretical value as was found for heterotrophic growth of Paracoccus denitrificans. We propose that metabolism in Paracoccus denitrificans is regulated on the level of phosphorylation in the site 1 and the site 3 region of the electron-transport chain.